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<script>
    // 1、求1 + 2！ + 3！ + 4！....+20!的值
	// - 2！ = 2 * 1
    // - 3！ = 3 * 2 * 1
    //n! = n*(n-1)!;
    var num = 1;
    for(var i=1;i<=20;i++){
        num = num*i;
    }
    console.log(num)
</script>